In spite of improvements in detection, treatment, and prevention techniques, dental caries remains a widely prevalent condition affecting people of all age groups. If not properly and promptly treated, caries can lead to permanent tooth damage and even to loss of teeth.
Traditional methods for caries detection include visual examination and tactile probing with a sharp dental explorer device, often assisted by radiographic (x-ray) imaging. Detection using these methods can be somewhat subjective, varying in accuracy due to many factors, including practitioner expertise, location of the infected site, extent of infection, viewing conditions, accuracy of x-ray equipment and processing, and other factors. There are also hazards associated with conventional detection techniques, including the risk of damaging weakened teeth and spreading infection with tactile methods as well as exposure to x-ray radiation. By the time caries is evident under visual and tactile examination, the disease is generally in an advanced stage, requiring a filling and, if not timely treated, possibly leading to tooth loss.
In response to the need for improved caries detection methods, there has been considerable interest in improved imaging techniques that do not employ x-rays. One method that has been commercialized employs fluorescence, caused when teeth are illuminated with high intensity blue light. This technique, termed quantitative light-induced fluorescence (QLF), operates on the principle that sound, healthy tooth enamel yields a higher intensity of fluorescence under excitation from some wavelengths than does de-mineralized enamel that has been damaged by caries infection. The strong correlation between mineral loss and loss of fluorescence for blue light excitation is then used to identify and assess carious areas of the tooth. A different relationship has been found for red light excitation, a region of the spectrum for which bacteria and bacterial by-products in carious regions absorb and fluoresce more pronouncedly than do healthy areas.
Among proposed solutions for optical detection of caries are the following:                U.S. Pat. No. 4,515,476 (Ingmar) discloses use of a laser for providing excitation energy that generates fluorescence at some other wavelength for locating carious areas.        U.S. Pat. No. 6,231,338 (de Josselin de Jong et al.) discloses an imaging apparatus for identifying dental caries using fluorescence detection.        U.S. Patent Application Publication No. 2004/0240716 (de Josselin de Jong et al.) discloses methods for improved image analysis for images obtained from fluorescing tissue.        U.S. Pat. No. 4,479,499 (Alfano) describes a method for using transillumination to detect caries based on the translucent properties of tooth structure.        
Among commercialized products for dental imaging using fluorescence behavior is the QLF Clinical System from Inspektor Research Systems BV, Amsterdam, The Netherlands. Using a different approach, the Diagnodent Laser Caries Detection Aid from KaVo Dental Corporation, Lake Zurich, Ill., detects caries activity monitoring the intensity of fluorescence of bacterial by-products under illumination from red light.
U.S. Patent Application Publication No. 2004/0202356 (Stookey et al.) describes mathematical processing of spectral changes in fluorescence in order to detect caries in different stages with improved accuracy. Acknowledging the difficulty of early detection when using spectral fluorescence measurements, the '2356 Stookey et al. disclosure describes approaches for enhancing the spectral values obtained, effecting a transformation of the spectral data that is adapted to the spectral response of the camera that obtains the fluorescent image.
While the disclosed methods and apparatus show promise in providing non-invasive, non-ionizing imaging methods for caries detection, there is still room for improvement. One recognized drawback with existing techniques that employ fluorescence imaging relates to image contrast. The image provided by fluorescence generation techniques such as QLF can be difficult to assess due to relatively poor contrast between healthy and infected areas. As noted in the '2356 Stookey et al. disclosure, spectral and intensity changes for incipient caries can be very slight, making it difficult to differentiate non-diseased tooth surface irregularities from incipient caries.
Overall, it is well-recognized that, with fluorescence techniques, the image contrast that is obtained corresponds to the severity of the condition. Accurate identification of caries using these techniques often requires that the condition be at a more advanced stage, beyond incipient or early caries, because the difference in fluorescence between carious and sound tooth structure is very small for caries at an early stage. In such cases, detection accuracy using fluorescence techniques may not show marked improvement over conventional methods. Because of this shortcoming, the use of fluorescence effects appears to have some practical limits that prevent accurate diagnosis of incipient caries. As a result, a caries condition may continue undetected until it is more serious, requiring a filling, for example.
Detection of caries at very early stages is of particular interest for preventive dentistry. As noted earlier, conventional techniques generally fail to detect caries at a stage at which the condition can be reversed. As a general rule of thumb, incipient caries is a lesion that has not penetrated substantially into the tooth enamel. Where such a caries lesion is identified before it threatens the dentin portion of the tooth, remineralization can often be accomplished, reversing the early damage and preventing the need for a filling. More advanced caries, however, grows increasingly more difficult to treat, most often requiring some type of filling or other type of intervention.
In order to take advantage of opportunities for non-invasive dental techniques to forestall caries, it is necessary that caries be detected at the onset. In many cases, as is acknowledged in the '2356 Stookey et al. disclosure, this level of detection has been found to be difficult to achieve using existing fluorescence imaging techniques, such as QLF. As a result, early caries can continue undetected, so that by the time positive detection is obtained, the opportunity for reversal using low-cost preventive measures can be lost.
One particular area of difficulty for caries detection relates to interproximal caries, that is, caries occurring on surfaces along the gap between adjacent teeth. In comparison with other portions of the tooth surface, such as with buccal or lingual portions, interproximal areas can be considerably more difficult to illuminate and to view. For viewing interproximal tooth tissue using conventional fluorescence imaging, it has been demonstrated that the excitation illumination that causes the fluorescence can be directed to the tooth from any of a range of angles within the same plane, that is, the plane containing the normals to the buccal, occlusal, and lingual surfaces, with little or no perceptible improvement between illumination at one angular orientation and another. This conclusion is reached, for example, in a paper by Buchalla, Lennon, van der Veen, and Stookey, entitled “Optimal Camera and Illumination Angulations for Detection of Interproximal Caries Using Quantitative Light-Induced Fluorescence” in Caries Research 2002, pp. 320-326. This paper suggests the importance of camera angle, but dismisses the impact of varying illumination angle for obtaining a suitable fluorescence image. Variation of illumination angle for outside the plane containing the normals to the buccal, occlusal, and lingual surfaces is not considered. Neither is the variation of illumination angle for non-fluorescence optical signal, such as reflectance, considered.
Interproximal caries is a highly prevalent form of dental caries that can be difficult to detect. Although imaging solutions such as those using fluorescence generation have shown some utility for detection of caries over areas of the tooth that can be more easily viewed, these solutions have shown little success in detecting interproximal caries. Thus, it can be seen that there is a need for a non-invasive, non-ionizing imaging method for caries detection that offers improved accuracy for detection of caries in its earlier stages and of interproximal caries.